Whenever it is desired to transfer or dissipate heat from one object to another by conduction, the critical element in the exchange is the joint between the two objects. The typical method of enhancing the heat exchange is to machine the interface surfaces of the two objects to high tolerances and hold them tightly together so that a relatively high percentage of the facing surface areas are in contact. Not only is this method expensive, but the thermal efficiency of the joint is a function of the smoothness of the surface finish and the amount of pressure applied thereacross. Even though the surface irregularities are microscopic and the pressure employed between the contacting surfaces large, only a relatively low thermal efficiency results. Known materials to enhance exchange across the joint include thermal greases which can reduce the thermal resistance across the joint by a factor of 2 or by making a metallic bond between the surfaces as by brazing. The thermal greases degrade with time as solvents evaporate with repeated opening and closing of the mechanical joint and have an affinity for contaminates such as dirt and dust so that a relatively inefficient heat transfer still results. Metallic bonds, on the other hand, can provide a thermal joint which has very little thermal resistance but which is very difficult to make and break in service. In some instances, as reported in the Journal of Heat Transfer, August 1963, Transactions of the ASME pp. 273-278 in an article entitled, "An Expermental Investigation of Free-Convection Heat Transfer From Rectangular-Fin Arrays" by K. E. Starner and H. N. M.sup.c Manus, Jr., liquid metal has been used in expermental situations to conduct heat from a heater to heat dissipating fins whose effectiveness is being investigated.
Airborne electronics for avionic systems produce very large heat loads and require sophisticated environmental control systems for cooling. In current experimental cooling systems solid state avionic components are attached to printed circuit boards which are provided with conductor strips. The strips may be small heat pipes which carry the heat from the components to a common sidewall which also may be a series of heat pipes which in turn are connected to a liquid cooled heat exchanger or cold wall. The connections between the solid state avionic components, the heat pipes, and the cold wall are all thermal joints which must be mechanically breakable to allow maintenance yet the prior art breakable thermal joints impede the heat transfer process.
In any heat exchange process, it is desirable to keep the temperature difference across a thermal joint between the component cooled and the coolant to a minimum for maximum efficiency. A reduced temperature difference possible because of reduced thermal resistance across the joint either allows the cooled components to operate at a reduced temperature or allows coolant to be supplied at a higher temperature. Therefore, the thermal interface resistance between the surfaces of each of the mechanical joints in a cooling system is critical to the overall heat exchange process of the system.
Whatever joint is supplied, it must be one that is capable of being removed frequently for servicing and repair or replacement. Also, the environmental conditions to which avionic components are exposed can be extremely harsh especially in terms of vibratory and inertial loading. Therefore, there has been a need for a relatively economic thermal joint with low thermal resistance which can be mechanically disconnected and reconnected on numerous occasions without degradation for use in vibratory or high G environments.